One type of rotary hammers comprise a motor which rotatingly drives a spindle supporting a chuck in which a drill bit can be held, while at the same time causing a piston tightly fitted within the spindle to execute a linear reciprocating motion within the spindle. This motion causes repeated compression of an air cushion between the piston and a ram slidably mounted within the spindle, which causes the ram in turn to execute reciprocating linear motion within the spindle and apply impacts to the drill bit via a beat piece.
The problem with such designs is that the frequency of impacts per revolution of the spindle is relatively small.
In an alternative design of hammer drill, the impacts which are imparted onto the drill bit are generated using two ratchet plates which interact with each other, one of which is fixed, the other rotating with the spindle, the vibrations generated by two ratchets being transferred to the drill bit. EP0613758 discloses such a design.
Though this design produces a high frequency of impacts per revolution of the spindle, the size of the impacts is relatively small.